1. Field of the Invention
This invention relates to a one-way clutch and a power transmission apparatus provided with this one-way clutch.
2. Related Art
An auxiliary equipment such as an alternator driven by rotation of a crankshaft of an engine is mounted on a vehicle such as an automobile. An endless belt is extended around a pulley fixed to a rotation shaft of the alternator and a pulley fixed to the crankshaft of the engine. The alternator uses rotating power of the engine as its drive source. In the case where the rotating power of the engine is thus used as the drive source of the alternator, the rotational speed of the crankshaft of the engine periodically varies, and therefore there are encountered a problem that slip occurs between the endless belt and the pulley of the alternator, so that abnormal noises are generated, and also the belt is worn.
Therefore, in such an alternator, it has been proposed to provide a one-way clutch between the rotation shaft and the pulley of the alternator in order to suppress the above problem. This one-way clutch as disclosed in JP-A-2002-130433 Publication comprises an inner ring fixed to the rotation shaft of the alternator, an outer ring fixed to the pulley, a plurality of rollers provided respectively in wedge-like spaces formed between the inner and outer rings, a cage holding these rollers, and springs urging the respective rollers in one direction. When the rotational speed of the outer ring becomes higher than that of the inner ring, so that the outer ring tends to rotate relative to the inner ring, the rollers bite into the respective wedge-like spaces, thereby establishing a locked state disenabling the relative rotation between the inner ring and the outer ring. On the other hand, when the rotational speed of the outer ring becomes lower than that of the inner ring, the biting engagement of the rollers is canceled, thereby establishing a free state allowing the relative rotation between the inner ring and the outer ring. Thus, the one-way clutch is switched between the locked state and the free state, and by doing so, the above-mentioned variation in the rotational speed can be absorbed, and the above problem is suppressed.
In the above one-way clutch used in the alternator or the like, in order to enhance the performance of biting engagement of the rollers with the inner and outer rings particularly in a low-temperature environment, it may be proposed to adopt means for using grease with excellent low-temperature characteristics as disclosed in the above JP-A-2002-130433 Publication, means decreasing the wedge angle between the contact surfaces of the roller and the outer ring, and means increasing the spring constant of the springs.
However, when the grease with the excellent low-temperature characteristics is used, the one-way clutch achieves an excellent performance in a low-temperature condition as when starting the engine, but subsequently in a high-temperature condition, the performance of the grease is lowered, and the generation of heat, etc., occur, and this leads to a fear that the lifetime of the one-way clutch may be shortened. In the case where the wedge angle between the contact surfaces of the roller and the outer ring is decreased, the contact pressure between these contact surfaces increases, so that heat is generated, and this lead to fears that the lifetime of the one-way clutch may be shortened and that the rollers may be bitingly held between the inner and outer rings to invite the locked state in a situation in which the inner and outer rings need to rotate relative to each other. Furthermore, in the case where the spring constant of the springs is increased, frictional heat between each roller and the inner and outer rings increases, and also the amount of the lubricant becomes insufficient, which leads to a fear that the lifetime of the one-way clutch may be shortened. Thus, although the conventional means can enhance the performance of biting engagement of the rollers with the inner and outer rings, these means lower other performances of the one-way clutch.
Furthermore, in recent years, engines have been enhanced in performance so as to produce a high output power, and as a result a variation in the rotational speed of a crankshaft of the engine has become larger. Therefore, the one-way clutch provided at an alternator or the like need to be smoothly and rapidly switched between the free state and the locked state. However, in the conventional one-way clutch, even when the above means are used, a film of grease exists between the contact surfaces of the roller and each of the inner and outer rings, and slip occurs between these contact surfaces, so that the required friction coefficient can not be obtained, and in some cases the above variation in the rotational speed can not be suitably absorbed. Particularly in a low-temperature environment, the viscosity of the grease increases, and a film of the grease formed between the contact surfaces, and slip is liable to occur, so that a tendency for failure to suitably absorb the variation in the rotational speed becomes strong.